NOTES AND EXTENSIONS.
P. j/. Brass and Gun-metal Founding.—With these
alloys a good height of pouring head is required, so as to cause
pressure and prevent porosity ia the casting. An ample number
of risers should also be provided to permit escape of air or gas,
and thus avoid honey-combing. No blackening is used, but the
mould is faced with very fine sand.
JP. 42. Steel Casting.—It having been noted that small
castings were more porous than large ones, the conclusion was
arrived at that it was of the utmost importance to keep the metal
at a very great heat until poured. The present method of pre-
venting blow-holes is to add silica-Spiegel (a combination of
silicon and pure cast iron) to the ladle while pouring, the better
plan being to apply it ia the molten condition. If this substance
be not thoroughly mixed we get 'hard' and 'soft' spots in the
casting, the former being due to accumulations of the silicon.
In order to keep the casting uniformly hot until the whole mould
be filled, the pouring gate should be chosen at the heaviest por-
tion of the casting; and small castings are preferably poured from
a small converfer to themselves, instead of from the refuse of a
large open-hearth melting. By facing the mould, where required,
witli ferro-raanganese or ferro-chromium, very hard surfaces are
obtained in those places, the latter substance giving the hardest